Before changes can begin to accumulate in any population, large or small, it must be isolated from other populations of the same species. Central to the concept of speciation, therefore, are the various mechanisms whereby populations can be held separate from one another.
Broadly speaking, there are two basic isolating mechanisms:extrinsic and intrinsic.
When two populations are separated by a physical barrier, such as a desert, canyon, sea, mountain range or forest, they are being isolated extrinsically, or by external means.
They physically cannot meet each other, so there is no chance of genes being passed between them. With two populations being separated in this way, different mutations accumulate in the two separate gene pools. Natural selection begins its differential action and eventually two separate species are formed.
Many times, however, two populations live virtually on each other's doorstep and share a common range within an ecosystem. They may see each other every day, but they never mate with one another.
Intrinsic reproductive isolating mechanisms, such as mechanical isolation (incompatibility of reproductive organs), behavioral isolation (differences in courting rituals), seasonal isolation (mating at different times of the year), and postmating sterility (hybrid offspring that are sterile) ensure that there is no exchange of genes between groups).